Unless protected, electronic signal processing apparatus, whether it be analog or digital in form, is sensitive to changes in supply voltage amplitude. Line voltage changes are often called "noise," especially when those changes occur at random. It is an integral part of many electrical design tasks either to eliminate noise from the power lines or to devise a system which is immune from the effects of such noise.
The problem occurs in many electronic applications. It is a particularly difficult problem in the design of radio receivers for mobile operation in motor vehicles. Generators and alternators, electric fuel pumps and windshield wipers, ignition systems, turning lights and automatic speed control servo systems, electric gauges and indicators, and voltage controllers all combine to impose transients and noise signals upon the basic unidirectional battery power source. That noise often appears as static in mobile radio receivers. The designers of entertainment radios ordinarily include noise limiting circuitry the effect of which is to permit the detection only of those radio signals whose amplitude is well above the noise level. The penality for that approach is that weaker radio signals go undetected along with elimination of the noise. That alternative is usually not available to designers of communication radio receivers and the receiving section of radio transceivers. Weak signal reception must be preserved in such radios, and the task has been to attempt to eliminate incoming noise, sometimes both at the antenna and at the input power line, but more often only at the power line.
The traditional approach has been to use brute force filtering. That involves including an inductive reactor in the ungrounded power supply line and by-pass capacitors between the ungrounded supply line and the vehicle ground. Such filtering may be coupled with the use of radio frequency noise suppression in the vehicle ignition system. Such suppression serves to diminish the radiation of radio frequency energy that might be picked up by the receiving antenna or that might be induced in the power leads.
Brute force filtering and ignition system shielding can, and usually do, have a substantial effect in diminishing noise and the effect is to permit reception of some of the weaker signals. However, the problem of noise entering mobile radios through power lines is still a very serious one in communication systems that are operated at high frequency or low power, or both. As a consequence, much room for improvement has remained in public safety, radio amateur, and citizens band mobile communications.